The present invention relates in general to multiplexing wireless broadcast signals among a plurality of antennas, and, more specifically, to a vehicular passive entry system driving selected ones of a plurality of antennas disposed in a vehicle.
It is well known in the automotive industry to provide for remote vehicle access, such as through the use of remote keyless entry (RKE) systems. RKE systems may be characterized as active or passive in nature. In an active system, a switch or pushbutton on a remote transmitter must be activated by an operator in order to have a desired remote function performed, such as locking or unlocking the vehicle doors. In contrast, a passive entry system does not require a pushbutton activation by an operator in order to have a desired remote function performed.
In remote entry systems, a portable transceiver is provided which is commonly referred to as a “fob” or a “card.” Such a fob or card may be attached to a key chain as a separate unit, or may be part of the head of an ignition key. The fob may function as both an active and a passive unit, i.e., having push buttons for user-initiated functions and having automatically operated circuitry to perform any of a variety of passive functions (such as unlocking a vehicle door, enabling the vehicle engine, and/or activating internal and/or external vehicle lights).
Passive entry systems include a transceiver in an electronic control module installed in the vehicle. The vehicle transceiver and/or control module is provided in communication with various vehicle devices in order to perform a variety of functions. For example, the vehicle transceiver and/or control module may be provided in communication with a door lock mechanism in order to unlock a vehicle door in response to an unlock request, or may be provided in communication with the vehicle engine in order to start the engine in response to an engine start request.
Passive entry communication operates over a much shorter range than RKE communication (e.g., 1 meter as opposed to 30 meters). Therefore, an LF signal (e.g., 134 kHz) is used for passive entry while a much higher frequency RF signal (e.g., 315 MHz or 433 MHz) is used for RKE since the LF signal decays over a shorter range. In addition, transponders operative at LF frequencies are readily available. As used herein, LF frequencies range from about 30 kHz to about 300 kHz. RF signals used in RKE systems are typically in the UHF band from about 300 MHz to about 3 GHz.
For a passive system, a sensor or switch may be provided in a vehicle door handle in order to provide the unlock request. More particularly, when the vehicle owner makes physical contact with the door handle, such as by grasping or pulling the handle, such a sensor provides the vehicle transceiver and/or control module with an indication of such contact. The vehicle transceiver and/or control module automatically transmits a passive entry challenge signal. Upon receipt of the challenge signal, the remote transceiver fob or card carried by the user determines if the challenge signal is valid and, if so, automatically transmits a response which includes a unique identification code of the fob. The vehicle transceiver and/or control module compares the identification code with the codes of authorized fobs and if a match is found then the control module generates a control signal that is transmitted to the door lock mechanism for use in unlocking the vehicle door.
In performing passive entry functions, it is often necessary to localize (i.e., determine the location of) the user carrying the fob in deciding whether a particular passive entry function should be performed. For example, when the vehicle door handle is activated to generate a door unlock request, the lock should actually be unlocked only if an authorized fob is located in the vehicle exterior. Otherwise, the vehicle door could be unlocked and opened by anyone outside the vehicle merely because an authorized user is present inside the vehicle. By way of another example, if a user activates a passive engine start switch inside the vehicle, the engine should actually be started only if an authorized user is present inside the vehicle.
One known method for determining the location of a fob is to employ separate vehicle antennas arranged to radiate primarily in the interior of the vehicle and primarily in the exterior of the vehicle, respectively. Multiple outside antennas may also be provided in order to detect whether the user is located at a particular vehicle door or at the trunk of the vehicle so that the proper door or trunk lid can be opened. In one particular type of system, the portable fob measures the received signal strength of the interrogation signals (i.e., challenge signals) from each of the respective antennas and then includes the signal strength information as part of a response message to the vehicle. The vehicle module then compares the signal strength at which the fob received the interior and exterior transmitted interrogation signals in determining whether the fob is present in the interior or exterior regions of the vehicle.
The vehicle transceiver functions as a base station including a single transmitter that is coupled to each of the antennas in the antenna array. In order to transmit from antennas individually, an antenna coupler or multiplexer is coupled between the transmitter and the antennas. Known multiplexers use a plurality of mechanical or semiconductor switches for directing the transmission signal to each antenna.
Typical mechanical switches utilize make-and-break contacts that are controlled by relays. After many operating cycles, the make-and-break contacts wear out and may become permanently open or permanently closed. These failures reduce. the expected operating lifetime of the passive entry system.
Semiconductor switches are not subject to contact wear, however other problems are encountered. Since the semiconductor switches are connected in series between the transmitter and antenna, they carry the full current applied to the antennas. Higher currents necessitate using higher cost semiconductors. Moreover, nonlinearity of the switches leads to signal distortion that adds harmonic content to the antenna signals. The harmonics degrade system perform making communications less reliable and reducing communication range.
Prior antenna coupling methods either pass the full signal to an antenna or block it. If it is desired to deliver some intermediate signal magnitude to any particular antenna, then the transmitter must be adapted to provide a variable output. The added cost and complexity of the transmitter has discouraged the introduction of functions depending upon a variable output, such as transmitting simultaneously from multiple antennas while equalizing their relative outputs to shape the coverage area of an RF broadcast.